Women infected with Zika can transmit it to their unborn child during pregnancy, but precisely how this happens has remained a mystery. Scientists have now revealed one potential route.

Among flaviviruses -- a group that includes Zika, yellow fever and West Nile virus -- it's rare for a virus to be transmitted from one generation to the next. Scientists believe that's because the organ connecting mother and fetus -- the placenta -- forms a barrier between the two individuals, separating their circulatory systems.

So how does Zika virus cross the placental barrier?

One possibility is that viruses slip through gaps created by damage or inflammation. Another is that Zika has a unique ability: it can cross the border by hiding inside a host cell. This hypothesis was inspired by pathology reports indicating that the virus infects placental cells.

Researchers at Emory University School of Medicine tested this idea using placentas donated by 5 healthy volunteers whose babies were born by Cesarean section. For the study, placental cells were grown in the lab and infected with a virus from Puerto Rico, which is closely related to the strain circulating in Brazil and responsible for the current Zika epidemic.

The Emory scientists found a rise in the activity of genes which produce antiviral molecules (indicating infection) in 'Hofbauer cells' of the fetal immune system. These are macrophages -- cells that swallow and digest foreign material -- that have direct access to fetal blood vessels. The invasions don't actually kill most cells, which is important because our defence system often uses dead cells as a signal to detect invaders and mount a response. The limited cell death therefore helps Zika evade detection.

More experiments (such as identifying which receptors allow Zika to enter cells) must confirm the results, but it seems the virus can invade and replicate within immune cells. By hijacking Hofbauer cells, virus particles could cross the placenta and then infect brain tissue in the developing fetus, leading to microcephaly.

But according to Mehul Suthar, the immunologist and assistant professor of pediatrics who led the new study, some donor cells were more susceptible to Zika than others. That means not every pregnant woman will transmit the virus.

"Host genetics and non-viral factors, including nutrition and microbiota, as well as timing may be influencing infectivity," said Suthar. "A better understanding of these factors could allow the design of preventive measures, and eventually antiviral therapies."

I'm an award-winning journalist with a PhD in evolutionary biology. I spent several years at Focus, the BBC's popular science and technology magazine, running the features section and writing about everything from gay genes and internet memes to the science of death and ori...